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Dr.-Ing. Jürgen Weber

"Prädiktive Direct Torque Control der Synchronmaschine" (2004)

The aim of this thesis was development and investigation of a high-dynamic torque control for servo drives with permanent magnet excited synchronous machines having constant sample and switching frequency. The designed control scheme has to be implementable on micro controller systems developed for fast fieldoriented drive control. For this purpose the "Direct Mean Torque Control - DMTC" turned out to be very suitable.

The advantage of DMTC compared to DTC in quasi-analogue implementation is, that if a motor with a small stray inductance is used, like in the given case, the torque ripple is much smaller. In addition, DMTC has the advantage over DTC that the switching frequency is constant. Compared to fieldoriented control the setup times of the torque are reduced. A disadvantage is the high harmonic content of the phase currents, in particular with high speeds and partial load.

Typical of the developed method is that within a control scheme one can determine the suitable voltage vector and switching time of the inverter for the next cycle in a predictive way during the momentary control cycle. In steady state the torque oscillates around the desired torque between the borders of a ripple band, resulting from the operation point. During a control cycle a sequence of an active voltage vector and a zero vector is applied to reach the upper border of the ripple band and return to the lower one. After a huge change of the desired torque an active voltage vector is applied to the machine for several control cycles to reach the steady state as fast as possible.

On the basis of the system equations of the permanent magnet excited synchronous machine and the bases of “Direct torque control – DTC” the needed equations to realise a direct "Direct Mean Torque Control- DMTC" were deduced. Beyond that a comparison of the deviations from the desired torque using a DTC and a DMTC for the drive used in this thesis took place, by which the advantages of a DMTC concerning the torque ripples were confirmed.
By using the derived equations an optimal strategy for the optimal choice of the in every sample time put on voltage vector is developed. Thereby the strategy to be used for the large signal behavior and the operation at the voltage limits is explained.

A digital simulation based on the developed DMTC control scheme and the data of the used drive confirms the theoretical considerations. Here the outstanding dynamics of the torque control showed up. For example, in standstill the rated load torque of the drive was reached within the first control cycle.

Furthermore it was shown that the prediction of the system variables by using the proposed simplifications as well as with high and low speed respectively at standstill deliver good results.

Following came experimentally examinations on a laboratory setup. As usual in servo-drive technology, it is assumed that the rotor position is measured. For the realization of the control algorithms a micro controller system developed for fast field-oriented drive controls was used. It was shown that the experimental results are quite similar to the results of the simulations. In particular the statements about the dynamic of the torque could be confirmed.
This control scheme represents, in particular if a high dynamic of the torque is demanded, an alternative to the remaining well-known control schemes of synchronous machines.